1. Field of the Invention
The present invention relates to a photovoltaic device and particularly to a photovoltaic device for partially transmitting incoming light.
2. Description of the Prior Art
Among photovoltaic devices for converting light energy to electric energy, an amorphous silicon solar cell having a large area can be easily manufactured at low cost. Such a solar cell in which a transparent front electrode layer, a photoelectric conversion semiconductor layer, and a metallic back electrode layer are stacked in order on a transparent insulating substrate is disclosed for example in U.S. Pat. No. 4,281,208. According to this patent, incoming light is transmitted to the semiconductor layer through the transparent insulating substrate and the transparent front electrode layer so that the incoming light is absorbed in the semiconductor layer to generate electron-hole pairs contributing to photoelectric conversion. Therefore, it is necessary to transmit the incoming light to the semiconductor layer as efficiently as possible, and for this reason, a front electrode of transparent conductive oxide (TCO) such as SnO.sub.2, In.sub.2 O.sub.3 or a mixture thereof, i.e., ITO is used.
On the other hand, the back electrode does not need to be transparent and accordingly it is formed of a single layer or a multiple layer including a metal layer of such as Al, Ag, TiAg, Ni, Au or Cr having lower sheet resistance compared with TCO.
If a photovoltaic device is used as a solar cell incorporated in a roof or a wall of a house, or a roof of a car, not as a power supply for a small-sized electronic device such as a portable electronic calculator, an electronic watch, or a portable radio, it is sometimes desired that sunlight be partially transmitted to the solar cell. For example, in a sun roof of a car, incoming light is partially intercepted by a mesh-like print pattern for example so that only about 1/10 of the incoming light, not all of it, is transmitted. An example of application of a solar cell to a car is described for example in Japanese Patent Laying-Open Gazette No. 50782/1983 and Japanese Patent Laying-Open Gazette No. 52884/1983.
A solar cell in which not only a front electrode but also a back electrode is formed of ITO so as to utilize light also from the back surface for photoelectric conversion is proposed in "S. 5-5, Problems in Development of Amorphous Si Solar Cells" by Konagai et al in National Convention Report IEE Japan, 1982. Such a solar cell is capable of transmitting light of a wavelength longer than about 650 nm which cannot be absorbed by a semiconductor layer.
However, although such a solar cell has transmissibility, it is necessary to use TCO having high resistance as a back electrode.